1. Field of the Invention
The present invention relates to an integrated process for producing a low sulfur, low olefin gasoline from a cracked naphtha, such as a full boiling range cracked naphtha stream. More particularly the stream is divided into at least two streams for individual treatment as required. Specifically the individual streams are hydrogenated, reacted to produce oxygenates and desulfurized.
2. Related Information
Petroleum distillate streams contain a variety of organic chemical components. Generally the streams are defined by their boiling ranges which determine the compositions. The processing of the streams also affects the composition. For instance, products from either catalytic cracking or thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials and polyunsaturated compounds (e.g., diolefins). Additionally, these components may be any of the various isomers of the compounds.
The composition of untreated naphtha as it comes from the crude still, or straight run naphtha, is primarily influenced by the crude source. Naphthas from paraffinic crude sources have more saturated straight chain or cyclic compounds. As a general rule most of the “sweet” (low sulfur) crudes and naphthas are paraffinic. The naphthenic crudes contain more unsaturates and cyclic and polycylic compounds. The higher sulfur content crudes tend to be naphthenic. Treatment of the different straight run naphthas may be slightly different depending upon their composition due to crude source.
Reformed naphtha or reformate generally requires no further treatment except perhaps distillation or solvent extraction for valuable aromatic product removal. Reformed naphthas have essentially no sulfur contaminants due to the severity of their pretreatment for the process and the process itself.
Cracked naphtha as it comes from the catalytic cracker has a relatively high octane number as a result of the olefinic and aromatic compounds contained therein. Frequently this fraction may contribute as much as half of the gasoline in the refinery pool together with a significant portion of the octane, and in some cases it may even contribute up to 90% of the gasoline in the refinery pool.
Catalytically cracked naphtha gasoline boiling range material currently forms a significant part (≈⅓) of the gasoline product pool in the United States and it provides the largest portion of the sulfur. The sulfur impurities may require removal, usually by hydrotreating, in order to comply with product specifications or to ensure compliance with environmental regulations. Now environmental concerns are requiring the removal of olefins. Both sulfur and olefin maximum contents are being lowered.
The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally copious quantities of hydrogen are included in the feed. The following equations illustrate the reactions in a typical HDS unit:RSH+H2RH+H2S  (1)RCl+H2RH+HCl  (2)RN+2H2RH+NH3  (3)ROOH+2H2RH+2H2O  (4)
Typical operating conditions for the HDS reactions are:
Temperature, ° F.600-780Pressure, psig 600-3000H2 recycle rate, SCF/bbl1500-3000Fresh H2 makeup, SCF/bbl 700-1000
The reaction of organic sulfur compounds in a refinery stream with hydrogen over a catalyst to form H2S is typically called hydrodesulfurization. Hydrotreating is a broader term which includes saturation of olefins and aromatics and the reaction of organic nitrogen compounds to form ammonia. However hydrodesulfurization is included and is sometimes simply referred to as hydrotreating.
After the hydrotreating is complete, the product may be fractionated or simply flashed to release the hydrogen sulfide and collect the now desulfurized naphtha.
The conditions of hydrotreating of the naphtha fraction to remove sulfur will also saturate some of the olefinic compounds in the fraction. However, this incidental olefin hydrogenation is usually not sufficient to meet the CARB requirements.
Since the olefins in the cracked naphtha are mainly in the low boiling fraction of these naphthas and the sulfur containing impurities tend to be concentrated in the high boiling fraction the most common method of treatment has been prefractionation prior to hydrotreating. The prefractionation produces a light boiling range naphtha which boils in the range of C5 to about 250° F. (C4 to about 250° F. if C4's are present in the naphtha stream) and a heavy boiling range naphtha which boils in the range of from about 250-475° F.
The predominant light or lower boiling sulfur compounds are mercaptans while the heavier or higher boiling compounds are thiophenes and other heterocyclic compounds. The separation by fractionation alone will not remove the mercaptans. However, in the past the mercaptans were frequently converted to sulfides by oxidative processes involving caustic washing. A combination oxidative conversion of the mercaptans followed by fractionation and hydrotreating of the heavier fraction is disclosed in U.S. Pat. No. 5,320,742.
The lighter fraction may be subjected to further separation to convert the C5 olefins (amylenes) which are useful in preparing valuable ethers.
More recently a new technology has allowed for the simultaneous treatment and fractionation of petroleum products, including naphtha, especially fluid catalytically cracked naphtha (FCC naphtha). See, for example, commonly owned U.S. Pat. Nos. 5,510,568; 5,597,476; 5,779,883; 5,807,477 and 6,083,378.
Full boiling range FCC naphtha has been hydrotreated in a splitter which contains a thioetherification catalyst in the upper portion. Mercaptans in the light fraction react with the diolefins contained therein (thioetherification) to produce higher boiling sulfides which are removed as bottoms along with the heavy (higher boiling) FCC naphtha. Similarly, the light fraction has been treated to saturate dienes. The bottoms are usually subjected to further hydrodesulfurization.
It is an advantage of the present invention that the sulfur may be removed from the light olefin portion of the stream to a heavier portion of the stream and the olefins converted to valuable octane enhancers. Substantially all of the sulfur in the heavier portion is converted to H2S by hydrodesulfurization and easily distilled away from the hydrocarbons.